This invention relates to an image forming apparatus such as a laser-beam printer or LED printer.
A known method of printing multivalued image data by a laser-beam printer or LED printer is an area tone method [hereinafter referred to as pulse-width modulation (PWM)] which, as shown in FIG. 69, involves further subdividing the smallest dot possessed by the printer, combining several of these to obtain the smallest unit (pixel) that represents density, and expressing tone in one pixel by the painted pattern.
FIG. 70 is a block diagram showing the configuration of a laser-beam printer for printing multivalued image data according to the prior art, and FIG. 71 is a block diagram showing the configuration of an LED printer according to the prior art.
In FIGS. 70 and 71, a printer controller 2603 within a printer 2602 receives image data (inclusive of photographic images and character images) output by a host computer 2601, which is an external device. The printer controller 2603 converts a character image into a prescribed bit-map signal (e.g., 3F[h] if black and 00[h] if white, where h represents a hexadecimal number). With regard to a photographic image, the printer controller 2603 sends six-bit image data to an image processor 2604 as a code signal indicative of density (white is 00[h], and the numerical value is enlarged as density increases, with 1F[h] representing black).
In the laser-beam printer, a laser driver 2605 shown in FIG. 70 controls the flashing of a semiconductor laser 2606 in dependence upon an output signal from the image processor 2604. In the case of the LED printer illustrated in FIG. 71, and LED driver 2605' causes an LED 2606' to flash in dependence upon an output signal from the image processor 2604.
FIG. 72 is a block diagram showing the image processor 2604 of the above-mentioned printer, which prints a half-tone image. Numeral 2701 denotes a .gamma.-correction table, which is constituted by a ROM or the like. Numeral 2702 denotes a two-bit main-scanning counter, 2703 a sub-scanning counter and 2704 a density-pattern generating table constituted by a ROM or RAM. Numeral 2705 denotes a parallel/serial converter which, in accordance with a clock having a frequency that is eight times that of an image clock VCLK, delivers eight-bit data output by the density-pattern generating table 2704.
The operation of the image processor constructed as set forth above will now be described. It will be assumed that the printer has a resolution of 600 dpi.
The printer controller (not shown in FIG. 72) sends the image clock VCLK, which is transmitted every dot of the 600 dpi, as well as five-bit multivalued image data transmitted in synchronization with the clock VCLK. The multivalued image data is subjected to an .gamma. correction by the .gamma.-correction table 2701 and is converted to five-bit image data, which enters addresses A0.about.A4 of the density-pattern generating table 2704. Meanwhile, the image clock VCLK is counted by the main-scanning counter 2702, the one-bit output thereof enters address A5 of the table 2704. Further, a horizontal synchronizing signal BD, which is sent from a printer engine whenever the semiconductor laser 2606 or LED 2606' makes a single scan, is counted by the sub-scanning counter 2703, the one-bit output whereof enters address A6 of the table 2704.
When the above-mentioned address enters the density-pattern generating table 2704, data of eight bits D0.about.D7, which have been loaded at this address in advance, are delivered by the table and then successively output starting from the MSB, by the parallel/serial converter 2705 in accordance with a clock VCLK.times.8, which has a frequency eight times that of the image clock VCLK. As a result, this operation forms the smallest or minimum unit, in which a total of four dots, namely two dots in the main scan and two dots in the sub-scan, represent density, with each dot being one dot of 600 dpi. Furthermore, one pixel has 32 subdivisions since one dot of 600 dpi is partitioned into eight parts. In other words, density is expressed by painting a certain number of the 32 subdivisions of one pixel in black.
FIG. 73 is an example of a density pattern. This is for a case in which multivalued image data indicates a density of 9/32.
In the conventional printer described above, however, a dot of 480 dpi is actually the smallest dot. If a half-tone image is printed by forming a pixel of dots having a resolution higher than this, a density irregularity in the main-scanning direction of the image becomes very conspicuous. The density irregularity is caused by a speed irregularity (hereinafter referred to as "pitch irregularity") in the paper conveyance system and drive system of the photosensitive drum in the printer.
In particular, since the spacing between LED's in an LED array is not perfectly uniform, density irregularity becomes conspicuous in the main-scanning direction. In case of 300 lines and 32 tones, the capabilities of the printer engine cannot be realized fully.
Density irregularity caused by pitch irregularity will be described.
FIG. 74 is a diagram illustrating a printing state for a case in which laser irradiation time is 50% in PWM performed in units of 300 dpi using the conventional printer. If the intervals between scanning lines of the laser are reduced owing to pitch irregularity in the printer, there is a rise in the energy distribution, which is obtained by combining the irradiation energy at each dot. At the same time, the energy that exceeds a development threshold value (namely the energy at which toner is capable of being affixed) spreads over a wider range, thereby broadening the area to which the toner is affixed and producing an increase in density. As a result, mutually adjacent pixels become connected in the main-scanning direction, as shown at portion A in FIG. 74, thereby causing a further increase in density.
If the intervals between scanning lines of the laser are increased owing to pitch irregularity in the printer, on the other hand, gaps are produced in the energy, as shown at portion B in FIG. 74, and the area to which the toner is affixed is reduced, thereby producing a decrease in density.
FIG. 75 is a diagram illustrating a printing state for a case in which laser irradiation time is 50% in PWM performed in units of 150 dpi. This diagram illustrates that an irregularity in printing density does not readily occur in PWM in units of 150 dpi even if the intervals between the laser scanning lines changes owing to pitch irregularity in the printer.
Further, in half-tone processing by modulation of laser luminous intensity a case in which laser luminous intensity is low so as to produce a low laser energy distribution and result in a reduced area of toner fixation per pixel is the equivalent of a reduction in the laser irradiation length w.
Thus, in a case where 150 dpi is adopted as one pixel, there tends to be little influence from pitch irregularity. However, resolution is low and character images do not appear sharp. If 300 dpi is adopted as one pixel, on the other hand, resolution is improved and character images are sharpened but the effects of pitch irregularity become a problem.